1. Field of the Invention
This invention relates generally to wireless communication systems, and more particularly to a method of frequency switching the preamble to minimize the channel estimation time in multiple input multiple output (MIMO) communication systems.
2. Description of the Prior Art
Increasing the capacity and reliability of communication systems is a focus driving much of systems technology growth. MIMO communication systems support this growth in the development of wireless networks. MIMO communication systems have been shown to provide improvements in both capacity and reliability over single-input single-output (SISO) communication systems. These MIMO communication systems commonly employ a block structure wherein a MIMO transmitter (which is a cooperating collection of N single-dimension transmitters) sends a vector of symbol information. This symbol vector may represent one or more coded or uncoded SISO data symbols. A MIMO receiver (which is a cooperating collection of M single-dimension receivers, where M is greater than or equal to N) receives one or more copies of this transmitted vector of symbol information. The performance of the entire communication system hinges on the ability of the receiver to find reliable estimates of the symbol vector that was sent by the transmitter. This necessitates that the MIMO receiver provide reliable channel estimates associated with transmissions from the MIMO transmitter.
A 2×2 MIMO communication system, for example, may transmit two independent and concurrent signals, employing two single-dimension transmitters having separate transmit antennas and two single-dimension receivers having separate receive antennas. Alternatively, the antennas could be derived from a single physical antenna that appropriately employs polarization. Two receive signals Y1(k), Y2(k) on the kth sub-carrier/tone following a Fast Fourier Transformation and assuming negligible inter-symbol interference may be written as:Y1(k)=H11(k)*X1(k)+H12(k)*X2(k)+n1(k)Y2(k)=H21(k)*X1(k)+H22(k)*X2(k)+n2(k)where X1(k) and X2(k) are two independent signals transmitted on the kth sub-carrier/tone from the first and second transmit antennas, respectively, and n1 and n2 are noises associated with the two receive signals. The term Hij(k), where i=1, 2 and j=1, 2, incorporates gain and phase distortion associated with symbols transmitted on the kth sub-carrier/tone from transmit antenna j to receive antenna i. The channel gain and phase terms Hij(k) may also include gain and phase distortions due to signal conditioning stages such as filters and other analog electronics. The receiver is required to provide estimates of the channel values Hij(k) to reliably decode the transmitted signals X1(k) and X2(k).
In order to estimate the channel coefficients Hij(k) at the receiver, the transmitter and the receiver employ training sequences. These training sequences are predetermined and known at both the transmitter and the receiver. In an IEEE 802.11(a) compliant system, a training sequence (called a long sequence) is employed as part of a preamble to the transmission of data. This long sequence involves the transmission of a known sequence of vector symbols, employing 52 excited tones (1 or −1), an unexcited tone (0) at DC and unexcited tones at each end of the spectrum, to provide a guard interval that is used to protect data tones from pass band filter effects. An appropriate calculation of individual channel coefficients (i.e., H11(k), H12(k), H21(k), H22(k)) may typically require a processor employing complex computations. Additionally, the level of computational complexity usually increases with an increase in the number of transmit antennas.
Time switched preamble designs commonly associated with MIMO communication systems, often result in long preamble times (i.e. preambles with many training symbols) which compromise data throughput.
Accordingly, what is needed in the art is a technique for switching the training symbols within the preamble associated with a MIMO communication system to ensure that data throughput is optimized.